Mapping Mountains – Hill Reclassifications – 200m Twmpau

Coed Bryn y Garth Lwyd (SH 725 576) – 200m Sub-Twmpau addition

There has been an addition of a hill to the list of 200m Twmpau, with the summit height, bwlch height and their locations, the drop and status of the hill derived from LIDAR analysis conducted by Myrddyn Phillips. 

LIDAR image of Coed Bryn y Garth Lwyd (SH 725 576)

The criteria for the list that this addition applies to are:

200m Twmpau – Welsh hills at or above 200m and below 300m in height that have 30m minimum drop, with an accompanying sub list entitled the 200m Sub-Twmpau, with the criteria for this sub category being all Welsh hills at or above 200m and below 300m in height with 20m or more and below 30m of drop.  The list is authored by Myrddyn Phillips, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward. 

200m Twmpau by Myrddyn Phillips

The name the hill is listed by is Coed Bryn y Garth Lwyd, and this was derived from the Ordnance Survey series of Six-Inch maps, and it is adjoined to the Moel Siabod group of hills, which are situated in the north-western part of North Wales (Region A, Sub-Region A2), and it is positioned with the A5 road to its north and the A4086 road to its west north-west, and has the village of Capel Curig towards the north-west.

When the original 200m height band of Welsh P30 hills were published on Geoff Crowder’s v-g.me website, this hill was not included in the accompanying Hills to be surveyed sub list, as it was considered not to meet the criteria then used for this sub category, as with no significant contours of note on the Ordnance Survey 1:50,000 Landranger and 1:25,000 Explorer map it was difficult to know whether any hill of note existed. 

Extract from the Ordnance Survey 1:25,000 Explorer map

Since the original publication of the Welsh P30 lists on Geoff Crowder’s v-g.me website there have been a number of maps made available online.  Some of these are historic such as the series of Six-Inch maps on the National Library of Scotland website.  Whilst others were digitally updated such as the Ordnance Survey Vector Map Local that was hosted on the Geograph website and which was entitled the Interactive Coverage Map, whilst others are current and digitally updated such as the interactive mapping on the Magic Maps and WalkLakes websites.

One of the mapping resources now available online is the WalkLakes website which hosts an interactive map originated from the Ordnance Survey Open Data programme.  This map has many spot heights not on other publicly available maps and for this hill it shows an uppermost 200m ring contour and bwlch contouring between 180m – 190m. 

Extract from the interactive mapping hosted on the WalkLakes website

However, it was not until LIDAR became available that the details for this hill could be accurately re-assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales.

Therefore, the addition of this hill to 200m Sub-Twmpau status is due to LIDAR analysis, resulting in a 202.2m summit height and a 179.7m bwlch height, with these values giving this hill 22.55m of drop, which is sufficient for it to be classified as a 200m Sub-Twmpau. 

 

The full details for the hill are: 

Group:  Moel Siabod 

Name:  Coed Bryn y Garth Lwyd 

OS 1:50,000 map:  115

Summit Height:  202.2m (LIDAR) 

Summit Grid Reference:  SH 72557 57676 (LIDAR) 

Bwlch Height:  179.7m (LIDAR) 

Bwlch Grid Reference:  SH 72372 57543 (LIDAR) 

Drop:  22.55m (LIDAR) 

 

Myrddyn Phillips (July 2025)